linux-kernel - Re: [patch] gpiolib: fix oops in gpio_get_value

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Message-ID: <20081016171505.GA5722@oksana.dev.rtsoft.ru>
Date:	Thu, 16 Oct 2008 21:15:05 +0400
From:	Anton Vorontsov <avorontsov@...mvista.com>
To:	David Brownell <david-b@...bell.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [patch] gpiolib: fix oops in gpio_get_value_cansleep()

On Thu, Oct 16, 2008 at 10:06:21AM -0700, David Brownell wrote:
> On Thursday 16 October 2008, Anton Vorontsov wrote:
> > > -     return chip->get(chip, gpio - chip->base);
> > > +     return chip->get ? chip->get(chip, gpio - chip->base) : 0;
> > 
> > Why don't we check the .set in the gpio_set_value? Because
> > we must always call gpio_direction_output()?
> 
> Yes.  The output driver needs to be explicitly enabled,
> to avoid misbehaving electic circuitry.
> 
> 
> > It is not exactly the 
> > same we work with the input direction.. is this documented anywhere?
> 
> In Documentation/gpio.txt since the very first versions.
> 
> See the section on "Spinlock-Safe GPIO access", where
> special cases for reading the value of output GPIOs are
> desribed.  Other aspects are mentioned in other spots.
> 
> For example, open drain signals -- as used with I2C and
> other protocols -- only drive the "low" signal level,
> and if code wants a "high" level it's got to verify that
> nobody else is driving that shared line to low.  By
> reading it back, even though it's configured as output.

I see. Much thanks for the explanations.

-- 
Anton Vorontsov
email: cbouatmailru@...il.com
irc://irc.freenode.net/bd2
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